1. Field of the Invention
The present invention relates to a method and system for rejuvenating pressurized fluid solvents used in cleaning fabrics, delicate electronic components, and similar sensitive substrates that may be adversely affected by soluble and insoluble contaminants entrained in the solvent. Particularly, the present invention is directed to a method and system for rejuvenating pressurized fluid solvents, such as liquid, subcritical, or supercritical carbon dioxide, without requiring 100% of the solvent to be vaporized for removal of contaminants, so as to reduce costs and adverse environmental impact.
2. Description of Related Art
A variety of methods and systems are known for cleaning fabrics, delicate electronic components, and similar sensitive substrates. These known methods and systems typically use water, perchloroethylene, petroleum, and other low pressure liquid solvents for cleaning the desired substrate.
Such conventional methods and systems generally have been considered satisfactory for their intended purpose. Recently, however, the desirability of employing these conventional methods and systems has been questioned due to environmental, hygienic, occupational hazard, and waste disposal concerns, among other things. For example, perchloroethylene frequently is used as a solvent to clean delicate substrates, such as garments and similar fabrics in a process referred to as "dry cleaning." Some locales require that the use and disposal of this solvent be regulated by environmental agencies, even when only small amounts of this solvent are to be introduced into waste streams. Such regulation results in increased costs to the user, which in turn, are passed to the ultimate consumer. It is therefore advantageous to provide a method and system for cleaning substrates utilizing a solvent having less adverse consequence than those solvents typically used.
In this regard, the use of alternative pressurized liquid or dense fluid solvents has been suggested for cleaning various substrates, wherein dense fluids are widely understood to encompass gases that are pressurized to either subcritical or supercritical conditions so as to achieve a liquid or a supercritical fluid having a density approaching that of a liquid. In particular, some patents have disclosed the use of a solvent such as carbon dioxide that is maintained in a liquid state or either a subcritical or supercritical condition for cleaning such substrates as clothing and precision metal parts.
As one example, expired U.S. Pat. No. 4,012,194 issued to Maffei discloses a garment cleaning process that uses liquid carbon dioxide. After passing through the garment, the liquid carbon dioxide solvent is circulated through an evaporator for removal of impurities, and then condensed by a refrigerated storage unit before being returned for further use.
Later patents modify the Maffei approach. Particularly, U.S. Pat. No. 5,316,591, issued to Chao et al., is directed to a method of cleaning a substrate by cavitating a liquified gas, such as liquid carbon dioxide. In the method disclosed by Chao et al., the substrate is placed in a cleaning chamber filled with the liquified gas, and a sonicating horn or similar cavitation-producing means is used to cavitate the liquified gas for a sufficient time to remove undesired material from the substrate. In one embodiment of Chao et al., the liquified gas is simply purged after the cleaning process is complete. In another embodiment, a closed loop is specified, such that all of the liquified gas is recirculated after first being purified by either vaporization, filtration, or an undefined combination of the two.
Rather than using liquified gas solvent, U.S. Pat. 5,013,366 issued to Jackson et al. is directed to a process for removing two or more contaminants from a substrate using phase shifting of dense gases. Specifically, Jackson et al. disclose storing a substrate in a pressure vessel filled with a liquified gas, and then varying the temperature within the vessel to shift the liquified gas between a liquid state and a supercritical state. The contaminated liquified gas is then exhausted to a separator and recycled to the vessel for repeated use. However, the structure and operation of the separator are not described.
Also issued to Jackson et al., U.S. Pat. No. 5,213,619 discloses a process for cleaning and sterilizing a material using one or more dense fluids mixed with chemical agents, and simultaneously subjected to both a high energy source of acoustic radiation and a nonuniform electrostatic energy field. No solvent purification method appears to be disclosed.
U.S. Pat. No. 5,267,455 and PCT publication WO 94/01613 to Dewees et al. are directed to a dry cleaning system that uses supercritical carbon dioxide for cleaning clothing. Once cleaning is accomplished by agitation within a vessel, all of the supercritical carbon dioxide within the vessel is drained to a vaporizer vessel for removal of entrained contaminates and then condensed for reuse.
U.S. Pat. No. 5,279,615 issued to Mitchell et al., as well as related foreign patent applications by the same inventors, are also directed to a method of cleaning fabric using dense carbon dioxide. Mitchell et al. further require the use of a nonpolar cleaning adjunct, however, to clean the fabric. After cleaning, the dense carbon dioxide is simply directed to an expansion vessel so that the extracted soils can be collected, while the carbon dioxide is apparently vented.
U.S. Pat. No. 5,313,965 issued to Palen is directed to a continuous cleaning system using a supercritical fluid. The system disclosed by Palen includes a main processing vessel having an entry airlock and an exit airlock. In this manner, purging of the supercritical fluid and decompression of the main processing vessel are not required. Although Palen states that the contaminated supercritical fluid may be processed in a conventional separator or recovery unit, no description of such separator or unit is provided.
Other patent publications that disclose cleaning processes using dense fluids include German patent application DE 3,904,514 and German patent DE 4,004,111. Both foreign publications disclose, among other things, purification by vaporization of all of the contaminated dense fluid prior to reuse.
As evident from the related art, conventional cleaning methods often require that the substrate to be cleaned is held within a bath of pressurized liquid or dense fluid solvent for a specific duration. This method may lead to recontamination of the substrate and degradation of efficiency since the contaminated solvent is not continuously purified or removed from the system.
Additionally, after cleaning is complete, conventional methods typically either vent all of the contaminated solvent to atmosphere or recycle 100% of the contaminated solvent for reuse after purification, such as by filtering or sequentially evaporating and condensing all of the solvent. It is believed, however, that efficiency is further degraded in each of these conventional cleaning methods. This is because it is costly to constantly replace or evaporate and condense all of the solvent that is used. The conventional methods of venting or evaporating and condensing all of the solvent also result in a complete loss of all co-solvents and additives that are used in the cleaning process, which further increases costs. With regard to the use of filtration alone, it is well known that this process allows soluble impurities to pass through the system and recontaminate the substrate.
There thus remains a need for an efficient and economic method and system for continuously rejuvenating pressurized liquid or dense fluid solvents that are used for cleaning fabrics, delicate electronic components, and similar sensitive substrates, without adversely impacting the environment or wasting expensive co-solvents and additives.